|Publication number||US6452524 B1|
|Application number||US 09/779,234|
|Publication date||Sep 17, 2002|
|Filing date||Feb 8, 2001|
|Priority date||Feb 8, 2001|
|Also published as||EP1374415A2, US20020130800, WO2002063777A2, WO2002063777A3|
|Publication number||09779234, 779234, US 6452524 B1, US 6452524B1, US-B1-6452524, US6452524 B1, US6452524B1|
|Inventors||Stephen Fraleigh, Douglas A. Cairns|
|Original Assignee||Ericsson Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Referenced by (21), Classifications (5), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to digital-to-analog converters, and, more particularly, to a delta-sigma digital-to-analog converter incorporating a multiplication function.
Digital-to-analog (D/A) and analog-to-digital (A/D) converters are widely used for converting electrical signals between digital and analog formats. Communication systems, for example, typically require extensive D/A and A/D conversion for performing a variety of signal processing functions. These converters may be implemented on an integrated circuit, such as an application specific integrated circuit (ASIC).
In an ASIC implementation, a converter may be combined with several other signal processing functions for performing the desired ASIC function. One common function implemented on such an ASIC is a multiplier function. There are several known techniques for providing a multiplier on an ASIC. While implementation of a multiplier is well understood and standard multiplier fabrication methods exist, the multiplier typically requires a relatively large area of the semiconductor chip on which the ASIC is formed. As a result, the multiplier typically consumes a significant amount of power.
A delta-sigma converter consistent with the invention includes a feed-forward path for receiving a feed-forward signal and providing an output signal; and a feedback path comprising a multiplying mapping function. The mapping function is configured to multiply the output signal based on at least one multiplication factor and to provide a feedback signal representative of the multiplied output signal. The feedback signal is combined with an input signal to provide the feed-forward signal. A method of multiplying a digital signal consistent with the invention includes coupling the digital signal as the input signal to a delta-sigma converter consistent with the invention.
A digital-to-analog converter consistent with the invention includes a delta-sigma converter configured to provide a digital output signal representative of an N-bit digital input signal multiplied based on at least one multiplication factor; and a digital to analog converter for providing an analog output signal representative of the digital output signal.
Advantages of the present invention will be apparent from the following detailed description of exemplary embodiments thereof, which description should be considered in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates an exemplary digital-to-analog converter consistent with the invention;
FIG. 2 illustrates an exemplary delta-sigma converter consistent with the invention; and
FIG. 3 illustrates a sampled-data equivalent circuit for an exemplary delta-sigma converter consistent with the invention.
With reference now to FIG. 1, there is illustrated an exemplary digital-to-analog converter 100 consistent with the invention. Those skilled in the art will recognize that a converter consistent with the invention may be incorporated into a variety of systems for achieving D/A conversion. A converter consistent with the invention may, for example, be provided in transmitters and receivers for a communication network, including wire-based and cellular or wireless networks using air interfaces, such as GSM, TDMA, CDMA or FDMA.
As shown, a D/A converter 100 consistent with the invention may include: a linear system 102 for receiving a multi-bit input signal x, i.e. a stream of digital words having a word length N; a multiplying delta-sigma converter 104 for receiving the output xa of the linear system; a digital-to-analog converter (DAC) 106 for receiving the output yq of the delta-sigma converter; and a low- pass filter 108 for receiving the output yd of the DAC. With the exception of the multiplying delta-sigma converter, the illustrated converter 100 includes elements conventionally provided in a delta-sigma-type D/A converter. A variety of configurations for the linear system 102, the DAC 106, and the low pass filter 108 will, therefore, be known to those skilled in the art.
The linear system 102 may, for example, be a conventional interpolator for changing the data rate of the incoming digital signal x and suppressing spectral replicas of the data in the frequency domain. In a case where an interpolator is not required, however, the linear system may simply comprise a direct connection of the digital input to the multiplying delta-sigma converter. In a known manner, a feed-forward path of the delta-sigma converter shortens the input word length, e.g. to a single bit, such that the quantization noise introduced by this process is outside of the signal baseband. As will be described in greater detail below, the delta-sigma converter is also configured to multiply the digital signal, thereby eliminating the need for a separate dedicated multiplier circuit.
The DAC 106, typically a one-bit DAC, may be of conventional construction and coupled to the output of the delta-sigma converter 104. The output yd of the DAC 106 is an analog representation of the digital input signal yq, including an amount of noise resulting from quantization error introduced by the delta-sigma converter. The noise may be significantly suppressed by the conventional low-pass filter 108 to provide an analog output y.
Turning now to FIG. 2, there is illustrated a block diagram of an exemplary multiplying delta-sigma converter 200 consistent with the invention. In general, the converter 200 incorporates a multiplying mapping function in the feedback path of a known delta-sigma converter configuration. For simplicity and ease of explanation, the present invention will be described herein in connection with a first-order delta-sigma converter configuration. It is to be understood, however, that the present invention is equally applicable to higher order delta sigma converters. Those skilled in the art will also recognize that the present invention is applicable to multi-bit, as well as single-bit, delta sigma converters.
In the illustrated exemplary embodiment, the converter 200 has a feed-forward path including a low pass filter 202 and a quantizer 204, and a feedback path including a multiplying digital-to-digital mapping function (MDDMF) 206. The low pass filter 202 may be provided in a variety of known configurations, and is typically an integrator. The output of the low-pass filter 202 is provided to the quantizer 204, which also has a known configuration. The quantizer is typically a two-level quantizer, but may be a multi-level quantizer. The quantized output yq is provided as the converter output and is fed back to the MDDMF 206.
In a manner to be described in more detail below, the MDDMF 206 is configured to map the quantized output, typically one bit, to a full-scale digital signal yFB representing the value of the quantized output. The signal yFB may have a word size, NFB, of at least the same number of bits, N, as the input signal xa. The MDDMF 206 also multiplies the quantized output based on a predetermined multiplication factor. The feedback signal yFB is combined with the input signal xa. This feedback through the MDDMF 206 forces the average value of the quantized output yq to track the average value of the input signal xa multiplied by the predetermined multiplication factor.
The operation of the MDDMF will be described in greater detail in connection with FIG. 3, which illustrates the sampled-data equivalent circuit for an ASIC implementation of an exemplary delta-sigma converter 300 consistent with the invention. As shown, the feed-forward section of the converter includes a discrete time integrator 302 and a quantizer 304 that introduces quantization error e[n]. The integrator 302 and the quantizer 304 function in a known manner.
In the illustrated embodiment, the MDDMF 306 maps the quantizer output yq to a full-scale digital feedback signal yFB[n] having the same number of bits as the input, e.g. a one bit quantizer output yq may be mapped to a 12-bit feedback value yFB[n]. The MDDMF 306 also multiplies the quantizer output yq based on a predefined multiplication factor α. Consistent with the present invention, the MDDMF 306 may be implemented as a lookup table indexed by the quantizer output yq[n]. For example, in an exemplary embodiment yFB[n] may be expressed as:
where yFB[n] is the NFB-bit feedback value at a discrete time n, yq[n] is the quantizer output, and α is the desired multiplication factor. A delta sigma converter consistent with the invention may thus be implemented for effecting both a D/A conversion and a digital multiplication through use of an appropriate MDDMF in the converter feedback path.
The multiplication factor a may be a single static value. It is to be understood, however, that a converter consistent with the invention may be configured to implement multiple and/or programmable values for α. In the case of programmable values, the mapping circuit may be configured to receive an input from an a selector 308 for defining one or more values for α. The selector may be integral with an IC on which the converter is implemented or may comprise an external connection. Those skilled in the art will recognize that a variety of configurations may be provided to manage multiple static or programmable a values. For example, lookup tables may be constructed for α0, α1, . . . αn and concatenated to form one large lookup table.
Those skilled in the art will also recognize that delta-sigma converters may produce artifacts if overdriven, i.e. if x[n]>yFB[n]. In the illustrated exemplary embodiment, to avoid such artifacts a may be selected such that 0<α≦1. For all values of α<1, the number of bits NFB will exceed N. The summing junction, the feedback path and the internal feed-forward paths should be configured to accommodate the worst case of the digital feedback word size.
The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention.
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|U.S. Classification||341/143, 341/155|
|Feb 8, 2001||AS||Assignment|
Owner name: ERICSSON INC., NORTH CAROLINA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:FRALEIGH, STEPHEN;CAIRNS, DOUGLAS A.;REEL/FRAME:011543/0812
Effective date: 20010207
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